1. Field of the Invention
This invention pertains to the field of gas riding face seals.
Gas turbine engines bleed off some of the compressed air from the primary gas path into so-called secondary flow circuits for various reasons, mainly to cool various components within the engine. This withdrawal of air is a parasitic loss to the engine thermodynamic cycle causing degradation in efficiency.
Secondary airflows are metered via air-to-air seals that are typically placed at interstage locations in the turbine engine. Relative velocities are high, typically about 150 meters/second (about 500 ft/sec) and up, and temperatures in the turbine section of the engine of about 425° C. (about 800° F.) and greater are typical. Since the beginning of the gas turbine, labyrinth seals have traditionally been used to seal these locations. Unfortunately, large thermal gradients, particularly during start up and shut down, result in considerable radial and axial excursions between the rotating and stationary parts of the seals. This makes it difficult to minimize operating clearances and so leakage through these seals, and the associated power loss, is usually significant.
It has been known for some time that if better seals were available engine performance could be substantially improved. See, for example, J. Munson and G. Pecht. “Development of Film Riding Face Seals for a Gas Turbine Engine.” STLE Tribology Transactions. V35 (1992). 1, 65-70. This reference shows that the use of just three advanced seals could reduce direct operating cost of a modem regional jet by almost 1%. This substantial benefit was the result of reduced fuel consumption and reductions in chargeable maintenance while producing the same power output at a lower turbine inlet temperature than an engine with conventional seals. In order to achieve these benefits it was necessary to place advanced mechanical seals very near to the blade/vane gaps in the high pressure turbine. Munson et al goes on to indicate that these locations are among the most difficult to seal because of the speed, temperatures, large excursions, and the inability to keep parts flat due to the large thermal gradients which characterize these locations. Munson et al provides a table of expected deflections and distortions at the three advanced seal locations along with speeds, temperatures, and differential pressure range.
2. Related Art
Efforts to provide improved seals for use in gas turbines and other applications have led to the production of abradable coatings for labyrinth seal stators and variations of labyrinth tooth geometry, and to the development of brush seals. These seals attempt to provide a labyrinth seal tooth with some compliance. The compliance allows the seals to track radial clearance excursions with only minimal wear of the seal. Leakage thus remains lower for a longer time relative to a labyrinth seal operating at the same location.
Over the past thirty years, several researchers tried to adapt mechanical face seals for use as advanced secondary air seals. Probably the earliest large effort in this direction is that described by L.S. Dobeck in an article entitled “Development of Mainshaft Seals for Advanced Air Breathing Propulsion Systems,” Pratt & Whitney Aircraft, NASA CR-121177 (1973). The focus of this effort was to modify the oil-cooled face seals already in use in engine bearing sumps to a configuration that did not require oil cooling. This program introduced the film-riding or gas-lubricated face seal. Later efforts followed, for example, see P. Lin-wander, “Development of Helicopter Engine Seals,” AVCO-Lycoming, NASA CR-134647 (1973) and “Self-Acting Seals for Helicopter Engines, AVCO-Lycoming. NASA CR-134940 (1975); M. O'Brien, “Development of a Short Length Self-Acting Seal,” AVCO-Lycoming. NASA CR-135159 (1976); and the 1992 J. Munson and G. Pecht article noted above. The work describes efforts to increase both the stiffness of the gas films and the demonstrated operating conditions. New lift features such as spiral grooves, etc., are described and new materials such as silicon carbide are introduced to overcome temperature limitations of carbon graphite.
More recently, J. F. Gardner et al U.S. Pat. No. 5.769,604 describes a double spiral groove hydrostatic-type seal. If one or both of the seal faces should experience a conical distortion in operation, these spiral grooves would tend to produce a moment on the seal faces in the opposite direction. To take advantage of this righting moment the stationary or primary seal ring has deliberately been made thin and flexible, the remainder of the seal follows typical face seal design practice. The intent of this design is to allow the hydrostatic seal to self-compensate for expected in-service conical distortion and thus potentially extend its useful operational envelope. The concept is currently under development.
The devices described in the aforementioned references describe hydrodynamic face seal designs. These rely primarily on the relative rotation of the seal faces to generate the lift force that separates the seal faces. The conclusion from review of this work is that the thin gas films that characterize this type of seal allow almost no distortion of the seal faces. In other words, even minute distortions in the seal faces must be prevented if adequate performance is to be achieved. In applications where this can be guaranteed successful applications result. For example, hydrodynamic designs have come to dominate the gas pipeline and process industry applications where distortion can be controlled, as described by P. E. Hesie and R. A. Peterson. “Mechanical Dry Seal Applied to Pipeline (Natural Gas) Centrifugal Compressors,” ASME-ASLE 1984 Joint Lubr. Conf., Preprint 84-GT-3. Where this cannot be guaranteed, such as inside a gas turbine engine, success has proved elusive.
Hydrostatic seals provide an alternative to hydrodynamic face seal designs. These only need an applied differential pressure. Hydrostatic designs work best with thin gas films, on the order of 0.0001 inch, but they can also operate with 10 times this film thickness. This increases the acceptable amount of distortion that the seal can tolerate without contact between the relatively rotating seal faces.
Turnquist, Tseng et al describes the development of a large hydrostatic face seal for use in an aircraft gas turbine engine in “Analysis and Full Scale Testing of an Aspirating Face Seal With Improved Flow Isolation”, 34th AIAA/ASME/SAE/ASEE Joint Propulsion Conference, Jul. 13-15. 1988, Cleveland, Ohio. The thick gas film allows the seal to cope with the expected distortion levels of the seal faces when operated in the engine. Leakage through this type of hydrostatic seal is much higher than that which would be expected from an equivalent diameter hydrodynamic face seal. However, it is pointed out that leakage is much lower than that which can be obtained from any other potential seal type, and is still expected to provide over 1% savings in engine specific fuel consumption (SFC). During engine start and shutdown, differential pressure is sometimes insufficient to separate the seal faces. To overcome this an “aspirating” labyrinth seal tooth has been applied in parallel with the seal. This allows the seal to remain “open” (non-contacting) until sufficient differential pressure is available to support the seal faces. This seal has undergone considerable rig development testing. It has met all of its test objectives. It is to be ground tested in a gas turbine engine in the near future.
Although promising for some gas turbine applications, hydrostatic seals cannot be utilized when the surface speed is high, typically over 305 meters/second (m/s) (1000 ft/s).
Allison Engine Company conducted an extensive literature survey of foil bearing capability prior to committing to the hybrid seal concept. Approximately 375 citations were reviewed covering the period from 1990 to the present. Although no references to foil face seals were reported, the literature survey revealed the existence of an extensive foil journal and thrust air bearing design, test, and manufacturing base. The great advantage of the foil design over the fixed geometry designs is the conforming nature of the foils. These have been shown to accommodate thermal and dynamic shaft and housing deflections. When used in journal bearing applications they have also demonstrated the ability to prevent half-speed whirl indicating that they are capable of providing stable operation.